Radio Science Experiments (RSEs) use radio links between
spacecraft and Earth to look for changes in the frequency as
well as round‐trip light time of signals to investigate
geophysical phenomena and for tests of fundamental
physics. BepiColombo and Juno are two space missions that will perform
RSEs, thanks to a very accurate tracking: to extract the desired
information, it is not sufficient to improve the precision of
observations, we need also to develop accurate mathematical models
making smaller the residuals, defined as the observed observables
minus the computed observables. However, sometimes, even highly
accurate mathematical models are not sufficient for the determination
of a given parameter with the required accuracy. In this paper, after
a brief summary of the mathematical models needed for top accuracy
RSEs, we will present two critical cases from the simulations of the
BepiColombo and Juno where also these accurate models may not be
sufficient to determine some quantities with the desired accuracy.

Radio Science Experiments (RSEs) use radio links between
spacecraft and Earth to look for changes in the frequency as
well as round‐trip light time of signals to investigate
geophysical phenomena and for tests of fundamental
physics. BepiColombo and Juno are two space missions that will perform
RSEs, thanks to a very accurate tracking: to extract the desired
information, it is not sufficient to improve the precision of
observations, we need also to develop accurate mathematical models
making smaller the residuals, defined as the observed observables
minus the computed observables. However, sometimes, even highly
accurate mathematical models are not sufficient for the determination
of a given parameter with the required accuracy. In this paper, after
a brief summary of the mathematical models needed for top accuracy
RSEs, we will present two critical cases from the simulations of the
BepiColombo and Juno where also these accurate models may not be
sufficient to determine some quantities with the desired accuracy.